Mechanisms of strain-induced grain structure formation-Reconsideration of recrystallization

研究了Cu、Al、Mg、Fe合金在低温和高温多向锻造和单道次压缩条件下大变形细晶组织的形成过程和机理。系统分析了变形特征与微观组织演化过程之间的关系，总结了主要研究结果：1。在临界应变以上，晶粒内部产生微剪切和扭结等多种变形带。随着应变的增加，晶粒的密度和边界的取向偏差增加，导致原有晶粒内部破碎，最终形成新的细晶粒。这种应变诱导晶粒形成的过程是由原位或连续动态再结晶控制的。在MDF过程中，变形带在三维空间中向各个方向发展，导致在大应变下形成细晶结构。这意味着MDF是形成细粒结构的有效方法之一。在较宽的温度区间内，新细晶粒的形成过程可分为三个阶段：(1)第1阶段（ε<2），形成常规位错亚结构并伴随变形带（DBs）的萌芽；(2)在第2阶段（2<ε<4），中等菌株的大尺度DBs发育后晶粒破碎，沿DBs形成新晶粒；(3)高应变阶段3 （ε>4）新晶粒组织的发育。这种应变诱导晶粒结构的退火不能通过常规的不连续再结晶（包括形核和新晶粒的长大）来控制，而可以通过原位或连续静态再结晶（晶粒长大伴随着恢复）来控制。

The process and mechanisms of fine-grained structure formation during severe large deformation was studied in multi-Directional forging (MDF) and a single pass compression at low and high temperatures by using Cu and Al, Mg and Fe alloys. The relationships between deformation characteristics and evolution process of microstructures were systematically analyzed, and the main results obtained were summarized as follows.1. Several kinds of deformation bands such as microshear and kink bands are generated in grain interiors in above a critical strain. The density and the misorientations of their boundaries increase with straining, leading to fragmentation of original grain interiors and finally formation of new fine-grains. The process of such strain-induced grain formation is concluded to be controlled by in-situ or continuous dynamic recrystallization.2. Deformation bands are developed in various directions in three-dimensional space during MDF, leading to formation of fine-grained structures in large strain. This means that MDF is one of effective methods for fine-grained structure formation.3. The process of new fine grain formation in a wide temperature interval can be classified into the three stages : (1) formation of conventional dislocation substructures accompanied with embryos of deformation bands (DBs) in stage 1 (ε<2) ; (2) development of large scale DBs followed by grain fragmentation, leading to new grain formation along DBs at moderate strains in stage 2 (2<ε<4) ; (3) development of new grained structures at high strains in stage 3 (ε>4).4. Annealing of such strain-induced grain structure can not be controlled by conventional discontinuous recrystallization including nucleation and growth of new grains, but also by in-situ or continuous static recrystallization, in which grain growth takes place accompanied with recovery.